WO02/068205, WO02/074548, WO2004/043704 and PCT/GB2004/003219, and also corresponding patent applications claiming the same priority dates, including U.S. patent applications Ser. Nos. 10/344,393 and 10/380,381 (the content of each of which is incorporated herein by reference), describe laser imaging and also materials that can be used for that purpose. Examples that are provided typically involve the use high energy lasers.
There are many attractions in using non-contact near-IR sources, in particular diode lasers, to generate images from coatings for applications such as variable information packaging. Favourable attributes of diode lasers such as economy, portability and ease of use, are attractive for current needs in the packaging industry, such as in-store labelling.
By incorporating, into ink formulations, materials which absorb radiation from far-IR to mid-IR sources such as heat (˜1 to 20 μm) and CO2 laser (˜10 μm), coatings have been produced which will generate a distinct coloured image on exposure to this wavelength of energy but not near-IR sources. By incorporating, into these same ink formulations, materials which absorb radiation from near-IR sources such as diode lasers (˜1 μm), coatings have been produced which will generate a distinct coloured image on exposure to near, mid or far-IR irradation.
Copper salts have been previously used (see U.S. Pat. No. 5,840,791A, US20030191223A and US20020016394A) as “laser light-active” compounds. They have been utilised in thermoplastic polymer mouldings, thermoplastic resin and thermoplastic polymer powder compositions etc., for laser marking of plastic components. Inorganic copper salts such as copper (II) hydroxy phosphate, copper (II) pyrophosphate and copper (II) sulphate, and organic copper salts such as copper (II) fumarate, copper (II) maleate and copper (II) oxalate, are known.